1. Field of the Invention
The present invention relates to a method of gamut mapping, and, more particularly, to a method of mapping a display device gamut onto a printing device gamut.
2. Description of the Related Art
The printing process often involves creating and viewing something on a display device before it is sent to a printing device. Consequently, it is desirable for the printing device output to look similar to the display device output. Since the physics behind the two devices is very different, many of the colors created on the display device cannot be created on the printing device and vice versa. Hence, the non-printable display device colors must be mapped to printable display device colors. Similarly, the mapping makes use of the printable colors that the display device does not use. This mapping process is referred to as xe2x80x9cgamut mapping.xe2x80x9d
The gamut of a device is defined as the set of all colors that can be produced by that device. The colors in the gamut are defined by using a three-dimensional device-independent color space such as CIELAB. A gamut is usually represented as a three-dimensional volume in CIELAB space. In CIELAB color space, luminance (lightness), hue (color) and chroma (saturation) are specified by three-dimensional coordinates (L*, a*, b*). CIE stands for Commission Internationale de l""Eclairage, an international body of color scientists. L describes relative lightness; A represents relative redness-greenness, and B represents relative yellowness-blueness. Luminance is specified by the L* value of the point of interest; hue is specified by the angle that is the arc tan of b*/a*; and chroma is specified by the distance of the point of interest from the L* axis, which can be calculated from the formula [(a*)2+(b*)2]1/2. FIG. 1 is a three-dimensional plot of the gamut of a typical computer monitor.
Gamut mapping is a process by which the gamut of one device is mapped onto the gamut of a different device. If the devices are similar, and hence have gamuts of similar shape and size, this process is relatively straightforward and usually consists of a one-to-one or near one-to-one mapping (by clipping the few out-of-gamut colors) from one gamut onto the other. However, when the devices are very different, which is the case for monitors and printers, the gamut mapping process is very problematic.
The gamut of a monitor and the gamut of a printer are very different because of the physics involved with the way the colors are generated. Colors generated on a monitor are made by emitting different intensities of three different colors of light (red, green, and blue). In the case of the printed output of a printer, in contrast, ambient light reflecting off of the printed output creates colors. The ink on the printed output absorbs different amounts of three different colors of light (red, green, and blue), thereby creating different colors. Monitors use what is referred to as xe2x80x9cadditive color mixingxe2x80x9d because they add or emit different intensities of three different colors of light. Printers use what is referred to as xe2x80x9csubtractive color mixingxe2x80x9d because the ink on the paper subtracts or absorbs different amounts of three different colors of light. Monitors are called RGB devices because they emit red, green, and blue light to make colors. Printers are called CMY devices because they use cyan, magenta, and yellow absorbers (ink) to make colors (cyan absorbs red, magenta absorbs green, and yellow absorbs blue).
FIGS. 2a, 2b, 3a, 3b, 4a and 4b illustrate the differences between a monitor gamut and a printer gamut. FIGS. 2a and 2b are projections of a monitor gamut plot and a printer gamut plot, respectively, onto the a*b* plane. FIGS. 3a and 3b are projections of a monitor gamut plot and a printer gamut plot, respectively, onto the L*a* plane. FIGS. 4a and 4b are projections of a monitor gamut plot and a printer gamut plot, respectively, onto the L*b* plane. As can be seen, the gamut of the monitor is more extensive than the gamut of the printer.
In a paper titled xe2x80x9cGamut Mapping Algorithms Based On Psychophysical Experimentxe2x80x9d (Proceedings of the 5th Color Imaging Conference, pp. 44-49) by Morovic and Luo, five gamut-mapping processes are evaluated via a psychophysical experiment. The five processes were reviewed and were found to have moderate to poor results. The main problem with these gamut-mapping processes is that they modify hue and lightness too great an amount. Changing the hue by any amount, or changing the lightness by too great an amount, destroys the similarity between the printer output and the display on the monitor.
Morovic and Luo describe a gamut mapping method called xe2x80x9cLinear Chroma Mapping and Lightness Range Mapping (CLLIN)xe2x80x9d. This method depends on first determining the cusps of the two different gamuts. The cusp of a gamut is defined as the boundary of the gamut in the a*b* projection and is generally taken to be the edges of the RGB color cube that go from red to yellow to green to cyan to blue to magenta and back to red in CIELAB space. Once the printer gamut cusp and the monitor gamut cusp have been determined, then at a given hue the chroma of the monitor is scaled using the following equation (1):
C*out=C*in(C*cusp(out)/C*cusp(in))
wherein C*in is the input monitor chroma, C*cusp(out) is the maximum value of the chroma of the printer cusp over the entire range of L* values, C*cusp(in) is the maximum value of the chroma of the monitor cusp over the entire range of L* values, and C*out is the remapped chroma (see FIG. 5). Next, lightness is mapped using the following equation (2):
L*out=L*outMin+(L*inxe2x88x92L*inMin)[(L*outMaxxe2x88x92L*outMin)/(L*inMaxxe2x88x92L*inMin)]
wherein L*in is the input monitor lightness, L*out is the remapped lightness (see FIG. 6), L*outMin is the minimum lightness of the printer at C*out, L*inMin is the minimum lightness of the monitor at C*out, L*outMax is a maximum lightness of the printer at C*out, and L*inMax, is a maximum lightness of the monitor at C*out.
The CLLIN gamut mapping method described in Morovic and Luo consists of two linear mapping steps and has some good attributes. The biggest problem with this method is that it tends to make the printer output too dark. This is because of the fact that the cusp of the printer gamut is much darker than the cusp of the monitor gamut.
What is needed in the art is a method of mapping a display device gamut onto a printing device gamut which does not result in a printer output that is too dark.
The present invention provides a non-linear gamut mapping process which preserves hue completely, preserves lightness for all but the more saturated or more chromatic colors, and performs most of the gamut mapping by changing chroma.
The invention comprises, in one form thereof, a method of mapping an input color gamut of an input device onto an output color gamut of an output device. A threshold chroma value in the input color gamut is established. For first input chroma values below the threshold chroma value, each first input chroma value is mapped onto a respective one of a plurality of first output chroma values such that a first linear relationship exists between the first input chroma values and the first output chroma values. Each corresponding first input lightness value is mapped onto a respective one of a plurality of first output lightness values such that a second linear relationship exists between the first input lightness values and the first output lightness values. For second input chroma values above the threshold chroma value, each second input chroma value is mapped onto a respective one of a plurality of second output chroma values such that a first non-linear relationship exists between the second input chroma values and the second output chroma values. Each corresponding second input lightness value is mapped onto a respective one of a plurality of second output lightness values such that a second non-linear relationship exists between the second input lightness values and the second output lightness values.
An advantage of the present invention is that the output of the printer is not too dark.